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    Voltage input to BLDC inverters on tethered drone?

    Hi,
    Regarding tethered drones such as the LIGH-T 4, which is rated at 1.2kW….
    https://elistair.com/ligh-t-tethered-station-for-drone/

    Do you know what voltage the BLDC motor inverters are likely to take?...presumably a lot more than a battery powered drone?
    Also, do you know what is the kind of power ranges that these tethered drones tends to be?

    presumably the voltage sent up the tether is high, but do you know how high?

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    Re: Voltage input to BLDC inverters on tethered drone?

    Tether wires appear lightweight and thin, implying low Amperes. I imagine it takes from 100 to 300 watts to lift a drone cam. And it seems convenient (as well as sensible marketing) to make the inverter interchangeable with mains AC.
    120V 60Hz or 220V 50Hz.


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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks, i suppose there is no power supply on the "tethered drone"?...just the BLDC inverters...which must us high voltage IGBTs?....

    Maybe somne just have a rectifier bridge and smoother to feed ~330VDC to the inverters?

    I wonder what switching frequency such inverters are likely to operate at?....

    The Elistair tethered droes are 1.2kw to 2.5kw, so i'm guessing they are fed by 1500vdc up the tether, which is directly switched by the inverter fed BLDCs?

    I am wondering if there is any trick here like doing current controlled BLDCs, instead of voltage controlled BLDCs?



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    Re: Voltage input to BLDC inverters on tethered drone?

    I wouldn't be surprised if the drone has its own step-down converter. High current is needed in R/C aircraft. 25 years ago electric planes typically had battery packs of 5 or 10 or even 20 'C' cells in series, to push several Amperes through the motors.

    1500 Watts is two horsepower. The Gossamer Condor flew on 1/2 HP, generated by one human in excellent athletic shape. I picture the tether wires as 18 or 20 gauge, a few hundred feet of run. It makes sense to carry 1500W as high voltage AC for the same reason electricity goes cross-country in that form.


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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks, i am wondering if it can be done the other way concerning the motors.......ie, high voltage and lower current?

    - - - Updated - - -

    Hi
    Would you agree that the old addage that “Motor torque is proportional to current” is not entirely true?....rather it is only true for an individual, particular motor.
    In other words, as long as I can provide the power, it doesn’t really matter what voltage I have to drive a BLDC with. Its not the current which produces the Torque…it’s the magnetic field….and I can have a large magnetic field with a low current if I have enough motor windings…..so Torque being proportional to current is a misleading generalisation. Would you agree?
    I this way, would you say that its very easy to make a 200W BLDC motor…..with a voltage input to its inverter of either 1000V, …or 20V? (assuming I have drive transistors available which can handle this high voltage)



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    Re: Voltage input to BLDC inverters on tethered drone?

    Hi,

    Would you agree that the old addage that “Motor torque is proportional to current” is not entirely true?
    I agree.
    You may apply a lot of DC current to a BLDC motor ... while generating zero torque.
    But if you apply a rotating field to a BLDC motor with the correct amplitude, and the motor is not stalled
    --> then the torque will be constant across the whole revolution. The motor coil current can not be constant, since it is AC.
    But the total power drawn (all three phases combined) is constant...and if this current is drawn from a DC bus (H bridge) with constant voltage, then the drawn current will be DC, too....and fairly proprtional to the torque.

    It's not that simple as with a DC motor .... and even there it is not perfectly proportional.

    Klaus
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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks, we are looking for a offtheshelf 200w-500w BLDC motor which takes Vin of 400V or more but cannot find one, do you know why this is?
    There are millions with voltages up to 48V, but almost none above this.



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    Re: Voltage input to BLDC inverters on tethered drone?

    Hi

    Which RPM?
    I used a milling motor for 400V, but also 400Hz which makes 24.000 RPM.
    I'm not sure... I assume this is an asynchronous milling motor.

    Klaus
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    Re: Voltage input to BLDC inverters on tethered drone?

    Quote Originally Posted by treez View Post
    Thanks, we are looking for a offtheshelf 200w-500w BLDC motor which takes Vin of 400V
    This calculates to average 1A current draw, 400 ohms impedance (in round numbers). That amount has to be achieved by reactive impedance (coils with large Henry value), or ohmic resistance (somewhere), or pulses of current at narrow duty cycle, etc. And/or possibly running at high frequency.

    There may be motors made this way but I suspect it's inefficient for aircraft. Motors propelling R/C aircraft are low ohms, say 1 to 10 range.


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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks,
    If a drone is up in the sky, and is then brought down to land.......then whilst it is coming down, what power are the motors running at?.....roughly what percentage of full power?
    I assume they are not generating back?...(like regenerative braking).



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    Re: Voltage input to BLDC inverters on tethered drone?

    I had some discussions with a tethered drone company and they were pushing about 1000V 1-2A DC up the cable. Resonant converters on each end.

    Drones are pretty high volume business now so it seems wise to piggyback on whatever common motors and controllers exist. Spend on the voltage conversion instead of a custom motor design I think.

    There are lots of bricks to go from 400 to 48V (Vicor)


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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks, i appreciate they wont ever generate back, and will always be running significant power, but wondered what generally the total power range (ie percentage of max range) is likely to be whilst in the air for a multi motor drone?



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    Re: Voltage input to BLDC inverters on tethered drone?

    For a 56kg tethered drone consuming 12.5kW and using an 800vdc feed from the ground based power supply…what voltage would you say was chosen for feeding to the BLDC motor inverters?
    (i was told it was "800v", but it may have been 800VAC of course ...but i somehow doubt it...unless it was 3-phase 800vac))
    Last edited by treez; 24th January 2020 at 23:50.



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    Re: Voltage input to BLDC inverters on tethered drone?

    12500W 800V works out to 15A. It seems reasonable considering lightweight lamp cord carries similar current levels. On the other hand if voltage were 400V then we're talking more than 30A which needs heavier cable. Etc.

    Replies at the link below suggest aluminum wire carrying high frequency high-voltage AC. Also whether balloons are a practical choice for lifting a drone-cam.

    "How to design drone power tether system? (high power with ultra light cable)"

    http://electronics.stackexchange.com...ra-light-cable


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    Re: Voltage input to BLDC inverters on tethered drone?

    I see people talking about high frequency AC...:

    Run an LLC converter with the primary in the base and the secondary and transformer in the drone with the cable forming part of the resonant L? I don't know off the top of my head what the impedance of such a long cable looks like to judge how feasible this is. Also the L and C of the cable might vary widely depending on whether it was spooled or not. I also don't know what the isolation requirements are in such a case. Should the drone (and therefore the power return) be ground referenced?


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    Re: Voltage input to BLDC inverters on tethered drone?

    good questions, we dont know though, since this has just been leaked.....weve only heard this little bit for now.
    I like the idea of the "spilt up LLC"......this was the modus operandi of isotera, when they operated. (they did a 50khz lighting bus).
    I think they got taken over by Greengage
    Last edited by treez; 25th January 2020 at 21:27.



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    Re: Voltage input to BLDC inverters on tethered drone?

    How did you advance from 1.2 kW to 12.5 kW drone power supply? I presume there are very few civil applications for this technology.


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    Re: Voltage input to BLDC inverters on tethered drone?

    I'd also look at Vicor, their isolated bus converters work bidirectionally. They don't say much about it but in the lower right of this they show the 48V<->800V (~1.kW) module being used to step power up and then down for "High-Voltage DC Transmission for Unmanned Vehicles".

    http://www.vicorpower.com/files/live...-BCM-VICOR.pdf

    Unfortunately they say nothing else about this application and instead say that its not qualified for continuous reverse operation (and imply it won't start up from 48V alone). So I think you'd have to ask about this. But I tend to assume Vicor is going to beat most options on size and weight.



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    Re: Voltage input to BLDC inverters on tethered drone?

    Quote Originally Posted by treez View Post
    For a 56kg tethered drone consuming 12.5kW and using an 800vdc...
    56kg is 123 lbs US. Is a manned craft in the works? Experimenters have done this and put up Youtube videos. Some appear genuine as their homemade drone lifts them in the air.

    Example, electric manned drone (untethered). He gives specs in the description. And advertises his e-book 'Manned multirotors'.

    https://youtu.be/RYLGhVPp8lw



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    Re: Voltage input to BLDC inverters on tethered drone?

    Thanks, Concensus seems to be saying that 48V is the right choice of offtheshelf motor to fly a 12.5kw, 56kg tehered drone.
    Would you agree that 13 * 48V, 10A BLDC motors would be the best way to fly such a drone?

    The we can look into asdf44's kindly shown Vicor modules (800 -> 48V) or perhaps a couple of paralleled PSFB's...or some open loop sync Bucks in parallel.......nothing has to be isolated in the drone...that would be done with the power supply on the ground.



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